One-step electrodeposition fabrication of Ni nanosheet ... · supercapacitor using the Ni 3 S 2...

mater.scichina.com link.springer.com Published online 30 October 2018 | https://doi.org/10.1007/s40843-018-9361-0Sci China Mater 2019, 62(5): 699–710

One-step electrodeposition fabrication of Ni3S2nanosheet arrays on Ni foam as an advancedelectrode for asymmetric supercapacitorsJiasheng Xu1*, Yudong Sun1, Mingjun Lu1, Lin Wang1, Jie Zhang1 and Xiaoyang Liu2*

ABSTRACT Ni3S2 nanosheet (NS) arrays on Ni foam werefabricated by a simple one-step electrodeposition strategy, andused as a kind of electrode material for asymmetric super-capacitors. The Ni3S2 NS arrays are interconnected, which canbe regarded as bridges between these individual nanoparticleunits. The electrochemical performances were evaluated bycyclic voltammetry and chronopotentiometry techniques in athree-electrode system. The Ni3S2 NS arrays display a specificcapacitance of 773.6 F g−1 at 1 A g−1, and excellent rate prop-erty of 84.3% at 10 A g−1. The performance of the Ni3S2 NSarrays was further investigated in an asymmetric super-capacitor for potential practical application. The asymmetricsupercapacitor using the Ni3S2 electrode and reduced gra-phene oxide electrode as positive and negative electrodes, re-spectively, exhibits an energy density of 41.2 W h kg−1 at1.6 kW kg−1. When up to 16 kW kg−1, it holds 25.3 W h kg−1.These excellent electrochemical performances are attributedto the improved electronic conductivity and rich redox reac-tion sites from Ni3S2 NS arrays. Our results indicate that theNi3S2 NS arrays have great potential for supercapacitors.

Keywords: nickel subsulfide, electrodeposition, nanosheet ar-rays, asymmetric supercapacitors

INTRODUCTIONThe consumption of petroleum and natgas fuels andemission of harmful soot gas lead to the urgent need forresearch and development of alternative and green energyconversion and storage devices [1–10]. Supercapacitors, analternative efficient and emerging energy storage system,have drawn intensive attention, owing to their safe op-eration mode, long cycle life and fast charge rate/highpower density [11–16]. Supercapacitors are generally di-

vided into two types⎯the pseudocapacitance and doubleelectric layered capacitance [17–22]. Pseudocapacitancetype electrode delivers a larger specific capacitance andhigher energy density from the rich Faradic redox reac-tions of metal oxides [23–27]. To date, various transitionmetal materials have been extensively explored anddemonstrated to be promising electrode materials foradvanced supercapacitors applications [28–33]. However,the relatively poor electrochemical capacity or lowconductivity of electrode materials still limits their large-scale practical applications for energy storage devices[34–36]. Thus, development of high-performance electrodematerials, such as large capacitance, high electrical con-ductivity and good electrochemical stability, is highly de-manded.

In recent years, transition metal sulfides have beenwidely explored and researched as the candidate of elec-trode materials [37–43]. Transition metal sulfides exhibitmuch smaller band gap compared with transition metaloxides, resulting in higher conductivity [44,45]. Thesubstitution of sulfur, possessing the lower electro-negativity, leads to a flexible phase structure, which canprevent the structure destruction and facilitate a pathwayfor the transport of electrons [46]. These excellentproperties endow them with better and high electricalconductivity and electrochemical performance, providingthe potential as electrode materials. Ni3S2 (nickel sub-sulfide) is regarded as an advanced energy storage ma-terial of supercapacitors because it possesses excellenttheoretical capacity, higher electrical conductivity andabundant reserves in nature [47,48]. The micro/nanos-tructured Ni3S2 electrodes with different morphologieshave been designed and fabricated through various ap-

1 Liaoning Province Key Laboratory for Synthesis and Application of Functional Compounds, College of Chemistry and Chemical Engineering,Center of Experiment Management, Bohai University, Jinzhou 121013, China.

2 State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China.* Corresponding authors (emails: [email protected] (Xu J); [email protected] (Li X))

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proaches. For example, Chen et al. [49] prepared self-supported Ni3S2 nanosheets (NSs) on Ni foam electrodethrough the wet-chemical method following a hydro-thermal process. Yang et al. [50] reported the mushroom-like Ni3S2/Ni foam through a solvothermal method. Huoet al. [51] prepared the NS Ni3S2/Ni foam electrodethrough a high-temperature hydrothermal method.

However, the above-mentioned fabricated processesrequire tedious chemical treatments, complex multi-stepand complicated procedures which are time-consumingand give rise to a lot of energy consumption and highfabrication cost. Furthermore, these processes are notsuitable for large-scale production, limiting the industrialdevelopment of Ni3S2 electrode for supercapacitors.Therefore, developing a low consumption, cost-effectiveand easy-control route to fabricate Ni3S2 electrode ma-terials and systematically studying their electrochemicalcapacitance is still a challenge for supercapacitors.

The electrodeposition method is widely used for thefabrication of nanomaterials, which can easily control thenanocrystal growth and their morphologies comparedwith chemical hydrothermal or solvothermal routes. Inthis work, we fabricated the interconnected Ni3S2 NSarrays on Ni foam through a simple one-step electro-deposition strategy. This process only includes an elec-trodeposition step for the electrolyte with the presence ofnickel chloride hexahydrate and thiourea. The relation-ship between microstructures and electrodeposition cy-cles was investigated. The effect of different morphologieson the electrochemical performances was also discussed.The Ni3S2 electrode displays a specific capacitance of773.6 F g−1 at 1 A g−1 and excellent rate performance of84.3% at 10 A g−1. The assembled NS@NF-20//rGO re-veals a power density of 41.2 W h kg−1 at 1.6 kW kg−1

(25.3 W h kg−1 at 16 kW kg−1). These results indicate thatthe Ni3S2 NS arrays will be a promising electrode materialfor energy conversion and storage devices.

EXPERIMENTAL SECTION

ReagentsThe chemicals were used without further purificationunless otherwise described. Nickel(II) chloride hexahy-drate (NiCl2⋅6H2O), potassium hydroxide (KOH) andthiourea (CH4N2S, TU) were purchased from TianjinGuangfu Technology Development Co. Ltd., Tianjin,China. N-methyl pyrrolidone (NMP) was purchased fromAladdin Biochemical Technology Co. Ltd., Shanghai,China. Poly(vinylidene fluoride) (PVDF), Ni foam, poly(vinyl alcohol) (PVA), acetylene black (AB) and the cel-

lulose separator were bought from Taiyuan Liyuan Li-thium Technology Co., Ltd., Taiyuan, China. Reducedgraphene oxide (rGO) was purchased from The SixthElement Materials Technology Co. Ltd., Changzhou,China. De-ionized water (18.3 MΩ cm) was obtained byMilli-Q water purification system.

Fabrication of Ni3S2 electrodesOne-step electrodeposition of the Ni3S2 electrodes wascarried out on a CHI660D electrochemical instrument(CH Instruments, Shanghai, China). The procedure wascontrolled through a CH Instruments Model softwarewithin the three-electrode cell system at 15°C. The silver/silver(I) chloride (Ag/AgCl) with saturated KCl solution,platinum plate (20 mm×20 mm×0.2 mm) and 100 mL ofde-ionized water containing 2 mmol NiCl2·6H2O and150 mmol CH4N2S were used as the reference electrode,counter electrode and electrolyte, respectively. A Ni foam(10 mm×20 mm×1 mm, more details are shown in TableS1 and Fig. S1, Supplementary information) was cleanedby sonication in 3 mol L−1 HCl, and then washed inethanol and de-ionized water. The cleaned Ni foam wasadopted as the work electrode. Cyclic voltammetry (CV)technique was carried out to fabricate Ni3S2 electrode at a10 mV s−1 sweep rate for 10, 20 and 40 sweep cycleswithin a potential window of −1.2 to 0.2 V in the de-position bath. These obtained Ni3S2 electrodes were wa-shed several times with de-ionized water, and then driedat 40°C in vacuum for 2 h. These Ni3S2 samples obtainedby the 10, 20 and 40 cycles were named as NS@NF-10,NS@NF-20 and NS@NF-40, respectively. The massloadings of the NS@NF-10, NS@NF-20 and NS@NF-40electrode were ca. 0.6, 1.2 and 3.5 mg cm−2, respectively.

Materials characterizationX-ray diffraction (XRD) patterns of Ni3S2 NS arrays onNi foam were measured using a diffractometer equippedwith Rigaku RAD-3C (Cu Kα, λ=1.5405 Å, 35 kV,20 mA, 2-Theta angles: 10°–70°). The morphologies andstructures were examined using a JEOL S-4800field-emission scanning electron microscope (FE-SEM)under the operating voltage of 3.0 kV, and a JEOLJEM-2100F transmission electron microscope (TEM)under the accelerating voltage of 200 kV. Energy dis-persive spectrometer (EDS) was also carried out usingthe JEOL S-4800 equipment with an EDS detector(Oxford). X-ray photoelectron spectroscopy (XPS) wasperformed on an ESCALB-MKII250 photoelectronspectrometer by a monochromatic radiation with Al Kαsource at 150 W.

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Electrochemical measurementsElectrochemical measurements were performed on aCHI660D instrument at 15°C. These Ni3S2 NS arrays onNi foam electrodes (NS@NF-10, NS@NF-20 and NS@NF-40) were directly used as the working electrodes. Plati-num plate (20 mm×20 mm×0.2 mm) was the counterelectrode. Hg/HgO (1 mol L−1 KOH) was the referenceelectrode. The 2 mol L−1 KOH was used as the electrolytefor this three-electrode cell. CV, chronopotentiometry(CP), cycling galvanostatic charge/discharge (GCD) andelectrochemical impedance spectroscopy (EIS) were exe-cuted to evaluate their electrochemical performances.

The specific capacitances of these electrodes andNS@NF-20//rGO asymmetric supercapacitor were cal-culated based on the GCD curves at various currentdensities using the following Equation (1) [52]:

C I tm V=×× , (1)

where C is the specific electrochemical capacitance(F g−1); I is the electric current (A) for the charge/dis-charge measurement; Δt is the time (s) during dischargemeasurements; m is the mass (g) of active material; ΔV isthe window voltage (V) in the CP measurement.

The NS@NF-20//rGO asymmetric supercapacitor wasassembled using NS@NF-20 as positive electrode, rGO asnegative electrode and PVA/KOH gel as electrolyte on aCHI660D workstation system. The fabrication details ofthe rGO electrode were given in the Supplementary in-formation (Page S3). 4.0 g of PVA powder was dissolvedinto 40 mL de-ionized water under vigorous magneticstirring at 90°C. After 1 h, 4.5 g KOH power was addedinto the as-dissolved PVA solution until the mixtureturned into the PVA/KOH gel. The NS@NF-20 electrode,the rGO electrode and the cellulose separator (more de-tails are shown in Table S2) were immersed into the as-prepared PVA/KOH gel for 10 min. Then, they were ta-ken out from the PVA/KOH gel and assembled to theNS@NF-20//rGO asymmetric supercapacitor. It was tes-ted on a CHI660D workstation.

The energy densities and power densities of theNS@NF-20//rGO asymmetric supercapacitor were cal-culated based on the following Equations (2) and (3):

E C V= 17.2 , (2)2

P Et=3600 , (3)

Where E is the energy density (W h kg−1); P is the powerdensity (W kg−1); C is the specific capacitance (F g−1) ofthe Ni3S2@Ni3S2//rGO asymmetric supercapacitor; ΔV is

the window voltage (V); t is the discharging time (s)during charge/discharge measurements.

RESULTS AND DISCUSSIONOne-step electrodeposition strategy for the fabrication ofNi3S2 electrodes is illustrated in Fig. 1. This electro-deposition strategy was executed in the three-electrodecell system as described in EXPERIMENTAL SECTION.CV was performed to deposit the Ni3S2 NS arrays on thesurface of Ni foam substrate within −1.2 to 0.2 V at10 mV s−1 for 10, 20 and 40 sweep cycles in the depositionbath. The relevant chemical reactions involved in theelectrodeposition process can be expressed as followingEquation (4) [53,54]:

2TU+3Ni +6e Ni S +2CN +2NH , (4)2+ 3 2 4+

The CV curves of these samples in the electrodepositionprocess are shown in Fig. S2. The Ni foam substrates areuniformly covered with the Ni3S2 NS arrays. The differentmicrostructures and morphologies of electrodes can beachieved at different sweep cycles (10, 20, 40 cycles).

The crystal phase and composition of Ni3S2 electrodewere characterized through XRD measurements and theXRD patterns are shown in Fig. 2. Two diffraction peakslocated at 45.1° and 52.5° are the characteristic peaks ofNi foam (marked by rectangular frames). Another eightdiffraction peaks located at 21.8°, 31.2°, 37.7°, 38.2°, 44.4°,49.7°, 50.1°, 55.2° and 55.4° are the characteristic peaks ofthe Ni3S2 NS arrays (marked by asterisks), which matchwell with the standard Ni3S2 phase (JCPDS card No. 44-1418). These diffraction peaks correspond to the reflec-tions of (101), (110), (003), (002), (113), (211) (122) and(300) planes of the Ni3S2 NS arrays, respectively. Notably,these diffraction peaks of the Ni3S2 NS arrays are weak,indicating the low crystallinity of the Ni3S2 NS arrays [55].

SEM images of these different morphological Ni3S2electrodes (NS@NF-10, NS@NF-20 and NS@NF-40) areshown in Fig. 3. Fig. 3a–d show the SEM images of theNS@NF-10 electrode. The panorama image of NS@NF-10(Fig. 3a) shows almost the same surface morphology aspure Ni foam (Fig. S1), and irregular Ni3S2 nanoparticlesare grown on the surface of Ni substrate (nucleation,Fig. 3b and c). From the high magnification SEM image(Fig. 3d), it is observed that some cross-linked andsmaller NSs are formed and grown on the Ni foam sub-strate. These sparse Ni3S2 nuclei on the surface of currentcollector substrate display an embryonic form for thehoneycomb-like Ni3S2 NSs.

The SEM images of NS@NF-20 are shown in Fig. 3e–h.The surface of Ni foam is different from that of NS@NF-



10. The enlarged SEM image in Fig. 3g clearly shows thatthe Ni3S2 NSs become much denser and larger to form thehoneycomb-like Ni3S2 NS arrays structure, which in-dicates the growth of Ni3S2 from nanoparticles to NSs onNi foam. Fig. 3h shows high-magnification SEM image ofNS@NF-20. The honeycomb-like Ni3S2 nanostructure iscomposed of interconnected Ni3S2 NSs, which can beregarded as the bridges between the individual nano-particle units, beneficial for improving structural stabilityof the NS arrays and enhancing the electrochemicalperformances. These Ni3S2 NSs possess a relatively uni-

form thickness of ~50 nm. The high magnification SEMimage shows that the rough surface of Ni3S2 NSs is cov-ered with smaller Ni3S2 NSs. The sufficient open spacebetween Ni3S2 NSs is also regarded as the electrolyte ionreservoir, which will be of benefit to the fast diffusion andtransfer of ions and electrons.

With the prolongation of the CV sweep cycles to 40, thesurface of Ni foam with Ni3S2 coating (Fig. 3j) is gettingrougher than those with less cycling time for electro-deposition (10 and 20 cycles). The transformation (fromNS to microsphere) of the surface morphology in theNS@NF-40 electrode is observed in Fig. 3k. To gain moredetail about the microsphere, Fig. 3l displays a high-re-solution SEM image which presents the presence of Ni3S2NSs on the surface of these microspheres. It is furtherobserved that the Ni3S2 microspheres consist of ag-gregates of many Ni3S2 NSs because the Ni3S2 NSs becomea larger and denser structure on surface of Ni foamsubstrate with the increase of CV sweep cycles. The Ni3S2microsphere growth possibly originates from the nucleion surface of Ni3S2 NSs. These microspheres result indecreased the space between interconnected Ni3S2 NSs.

The structure and morphology of Ni3S2 NSs are alsocharacterized by TEM. Fig. 4 shows the typical TEMimages of Ni3S2 NSs scratched from the NS@NF-20electrode. Fig. 4a displays the overall contour of Ni3S2 NSswith bending and wrinkles. The enlarged TEM images inFig. 4b and c reveal that the thickness of Ni3S2 NSs is ca.30 nm. It appears that Ni3S2 NSs are also wrapped withtiny NSs (the regions marked by red rectangles and red

Figure 1 Schematic illustrations of the electrodeposition process for the Ni3S2 NS arrays on Ni foam substrate.

Figure 2 XRD patterns of the Ni3S2 NS arrays on Ni foam. The dif-fraction peaks of Ni3S2 are marked by asterisks and those of Ni foam aremarked by rectangular frame. Several vertical lines at the bottom are thestandard diffraction peaks of Ni3S2 from JCPDS card No. 44-1418.



dashed lines show the core-shell structure). The tiny Ni3S2NSs are homogeneously distributed on the surface oflarge-sized Ni3S2 NSs to form Ni3S2@Ni3S2 core-shellstructure, which is consistent with the correspondingSEM images. This Ni3S2@Ni3S2 core-shell structure canincrease the surface area more than single Ni3S2 NSs,which shortens the distance of ion/electron transporta-tion and enhance the electrochemical performance. Ascan be observed in the high magnification TEM image ofNi3S2 NSs (Fig. 4d), the tiny Ni3S2 nuclei are formed andanchored over the surface of Ni3S2 NSs (the regionsmarked by blue circles). These nuclei play a vital role inthe growing process for Ni3S2 NS arrays and the Ni3S2@Ni3S2 core-shell structure. The tiny Ni3S2 nuclei have anarrow distribution in size of ca. 2 nm to 5 nm. Fig. 4eand f display the high-resolution TEM (HRTEM) imagesof Ni3S2 NSs. The characteristic lattice spacing of Ni3S2NSs are measured to be 2.87, 4.06 and 2.38 Å, which wellmatch with the spacing of the (110), (101) and (003)planes of Ni3S2 NSs, respectively. Inset in Fig. 4f shows

corresponding selected area electron diffraction (SAED)patterns of Ni3S2 NSs, revealing that the diffraction ringsare well indexed as those of the (110), (202) and (211)planes of the Ni3S2 NSs, respectively. These character-izations are well consistent with XRD patterns.

The electrodeposition growth of Ni3S2 can be dividedinto two steps: the initial Ni3S2 nucleation and the sub-sequent growth of the individual Ni3S2 nucleus. At first,the formation of tiny Ni3S2 nuclei, also called as seedcrystals, occurs on surface of Ni foam substrate with highdensity, which play a vital role in growth of Ni3S2 NSarrays. Subsequently, as the CV sweep cycle increases, theNi3S2 nuclei grow to Ni3S2 NS arrays. When increasingthe CV sweep cycles to 40, the Ni3S2 microspheres areassembled from the dense Ni3S2 NSs, and Ni foam isalmost fully coated and cladded by denser Ni3S2 NSs.

EDS mappings of the Ni3S2 sample are shown in Fig. 5and the corresponding mappings of Ni and S clearly in-dicate the homogeneous distribution of Ni and S ele-ments. To further analyze their elemental compositions

Figure 3 SEM images of Ni3S2 NS arrays on Ni foam with different morphologies by altering the sweep cycles in the electrodeposition process. (a–d)Low- and high-magnification SEM images of Ni3S2 NS arrays after 10 cycles (NS@NF-10), scale bars = 20 μm, 10 μm, 1 μm and 200 nm, respectively.(e–h) Low- and high-magnification SEM images of Ni3S2 NS arrays after 20 cycles (NS@NF-20), scale bars = 30 μm, 10 μm, 1 μm and 100 nm,respectively. (i–l) Low- and high-magnification SEM images of Ni3S2 sample after 40 cycles (NS@NF-40), scale bars = 20 μm, 10 μm, 1 μm and200 nm, respectively.



and valence states, NS@NF-20 electrode was character-ized by XPS. Fig. 6a displays the full survey XPS spectrumof the NS@NF-20 electrode, which shows the elements ofS and Ni. Other peaks of the C, N and O elements aremost likely from the air [56,57]. Fig. 6b shows the Nispectrum of NS@NF-20 electrode, which consists of twopeaks at 856.3 and 874.0 eV with a spin orbit energy se-paration of 17.7 eV, corresponding to Ni 2p3/2 and Ni2p1/2 components of the Ni

2+ in Ni3S2, respectively [58].

Two peaks located at 861.9 and 880.2 eV are the satellite(Sat.) peaks of the Ni 2p3/2 and Ni 2p1/2, respectively [59].Fig. 6c displays S spectrum of NS@NF-20 electrode. The Speaks observed at 162.7 and 164.0 eV correspond to S2p1/2 and S 2p3/2, respectively, consistent with the nickelsubsulfide in the previous reports [58–60]. The S peak at168.7 eV is attributed to the presence of sulfate radicalion, indicating that partial S2− are likely oxidized to sulfatein air [38,60].

Typical CV curves of the NS@NF-20 electrode within−0.2 to 0.9 V at various sweep rates in the range of 5–100mV s–1 are shown in Fig. 7a. CV curves of this NS@NF-20electrode with a couple of redox peaks are attributed toreversible Faradic behavior of electrode in alkaline elec-trolyte as expressed in Equation (5) [61]:

Ni S + 3OH Ni S (OH) + 3e . (5)3 2 3 2 3

The shape of CV curves of NS@NF-20 electrode almostdoes not change when the sweep rate increases, indicating

Figure 4 (a–d) The typical TEM images with different resolution of theNi3S2 NSs, scale bars = 100, 50, 50 and 20 nm, respectively. (e, f) High-resolution TEM images of the Ni3S2 NSs, scale bar = 5 nm. The insetshows the corresponding selected area electron diffraction (SAED)pattern of the Ni3S2 NSs. (The regions marked by red rectangles and reddashed lines show the core-shell structure; the regions marked by bluecircles show some nuclei on the surface of NS).

Figure 5 (a) SEM image of the Ni3S2 NS arrays. (b) Overlapped EDSmapping of the Ni3S2 sample taken from the rectangular frame in (a). (c)S mapping. (d) Ni mapping.

Figure 6 (a) XPS full spectra of Ni3S2 NS arrays. (b) Ni 2p spectrum. (c) S 2p spectrum.



that the NS@NF-20 electrode possesses low resistancebetween electrode and electrolyte, fast Faradic reactionsas well as good rate capability [62]. The anodic peaks ofCV curves shift to negative potential and the cathodicpeaks shift to positive potential. The absolute values ofthe response currents of the anodic and cathodic peakscorrespondingly increase with the increase of sweep rate.These results may be owing to the polarization effect.

Fig. 7b displays the GCD curves of NS@NF-20 elec-trode at 1–10 A g−1. According to the Equation (1), thespecific capacitances of NS@NF-20 are calculated to be773.6, 755.2, 734.4, 676.8, 662.4 and 652.5 F g−1 at 1, 2, 4,6, 8 and 10 A g−1, respectively. Above calculated specificcapacitances of NS@NF-20 electrode are shown in Fig. 7e.The NS@NF-20 electrode possesses a larger specific ca-pacitance compared with the NS@NF-10 and NS@NF-40electrodes. The pure Ni foam as working electrode is alsotested at the same condition, indicting a negligible ca-pacitance of Ni foam (Fig. S3).

To understand the effect of electrodeposition sweepcycles on the electrochemical performances, the NS@NF-10 and NS@NF-40 electrodes were also investigated byCV and CP tests, and compared with the NS@NF-10electrode. Fig. 7c shows the CV curves of the three elec-trodes at 5 mV s−1. We can observe that all the CV curves

have redox peaks, but the response currents of redoxpeaks and the enclosed areas of CV curves are differentwith increase of electrodeposition cycles. The NS@NF-20electrode has the highest response current and the largestCV enclosed area, indicating a higher specific capacitanceof NS@NF-20 electrode than that of the NS@NF-10 andNS@NF-40 electrodes. Figs S4 and S5 show the integratedCV curves of NS@NF-10 and NS@NF-40 electrodes atvarious current densities.

Fig. 7d displays the discharging curves of the NS@NF-10, NS@NF-20 and NS@NF-40 electrodes at 1 A g−1.These discharging curves display two distinct voltageplateaus, which are attributed to the pseudocapacitivebehavior arising from reversible Faradic redox. The dis-charge curves do not form a straight line, owing to theircharge and discharge plateaus. The integrated GCDcurves of NS@NF-10 and NS@NF-40 electrodes at var-ious sweep rates are shown in Figs S6 and S7. From theGCD curves, the discharge time of NS@NF-20 electrodeis 386.8 s, which is larger than those of NS@NF-10(204.7 s) and NS@NF-40 electrodes (144.4 s), indicatingthat the specific capacitance of NS@NF-20 electrode(773.6 F g−1) is larger than those of NS@NF-10(409.4 F g−1) and NS@NF-40 (323.2 F g−1) electrodes at1 A g−1. For comparison, the specific capacitances of the

Figure 7 (a) CV curves of NS@NF-20 electrode at various sweep rates. (b) GCD curves of NS@NF-20 at various current densities. (c) ComparativeCV curves of NS@NF-10, NS@NF-20 and NS@NF-40 electrodes at a sweep rate of 5 mV s−1. (d) GCD curves of the three electrodes at a currentdensity of 1 A g−1. (e) The specific capacitances of the three electrodes as a function of the current density. (f) Nyquist plots of the three electrodes. Theinset shows the enlarged plots in the high frequency part.



three electrodes at 1, 2, 4, 6, 8 and 10 A g−1 are shown inFig. 7e, which indicates that the specific capacitances ofNS@NF-20 electrode are larger than those of NS@NF-10and NS@NF-40 electrodes at 1, 2, 4, 6, 8 and 10 A g−1.The specific capacitance of NS@NF-20 electrode is su-perior to those of the nickel sulfide based materials inpreviously reported literatures (Table S3).

Fig. 7e also displays the excellent rate capability ofNS@NF-10, NS@NF-20 and NS@NF-40 electrodes. It isobserved that with increasing from 1 to 10 A g−1, thespecific capacitance decreases from 406.3 to 250 F g−1 forNS@NF-10 electrode, from 773.6 to 652.5 F g−1 forNS@NF-20, from 323.2 to 170.0 F g−1 for [email protected] up to 10 A g−1 from 1 A g−1, the specific capaci-tance of NS@NF-20 electrode still retains 773.6 F g−1 withthe good rate capacity of 84.3%. Rate capacities of theNS@NF-10 and NS@NF-40 electrodes are 61.5% and52.6%, respectively, indicating the Ni3S2 electrode pos-sesses high reversibility of the redox charge storage re-actions. Furthermore, the volumetric capacitances of theNS@NF-20 electrode were also calculated (Fig. S8). Thecycling stability is also an important parameter for su-percapacitors. Fig. S9 shows the cycling GCD measure-ment of NS@NF-20 electrode, which retains 81.7% ofinitial capacitance after 5,000 cycles at 5 A g−1, indicatinga fine long-term stability of NS@NF-20 electrode.

Fig. 7f displays the Nyquist plots of NS@NF-10,NS@NF-20 and NS@NF-40 electrodes in 0.01 Hz to100 kHz. EIS spectra are approximately divided into tworegions: the semicircles in the high-frequency range andthe straight lines in the low-frequency range. In the high-frequency part, the intercept at the real axis (Zʹ) re-presents the intrinsic resistance of electrodes (Rs). Rsconsists of intrinsic resistance of active material, ionicresistance of electrolyte as well as contact resistance be-tween active material and Ni foam substrate [63]. Theinset in Fig. 7f shows that the values of Rs of NS@NF-10,NS@NF-20 and NS@NF-40 electrodes are 0.86, 0.89, and0.93 Ω, respectively. These Ni3S2 electrodes exhibit lowintrinsic resistance, indicating the good conductivity ofthe Ni3S2 NS arrays with excellent pathways for electrontransport. In the high-frequency range, a semicircle isobserved and its diameter represents charge-transfer re-sistance (Rct) between electrode and electrolyte associatedwith reversible Faradaic reactions [53]. The diameter ofNyquist plot of NS@NF-20 electrode is smaller than thatof NS@NF-10 and NS@NF-20 electrodes, indicating thefast charge-transfer kinetics of NS@NF-20 electrode. Inthe low-frequency range, the slope indicates the diffusionbehavior of electrode materials. The bigger the linear

slope, the faster it is for the diffusion of ion/electron.These large-slope lines reveal that the Ni3S2 electrodeshave fast electron transport performances due to theirhigh conductivity. The results based on EIS analysis showthat the Ni3S2 NS arrays possess high ionic and electronicconductivities, which can effectively reduce resistancesand provide a highway for ion and electron transfer inreversible Faradaic reactions.

To further evaluate the practical application perfor-mances of Ni3S2 NS arrays, the NS@NF-20//rGO asym-metric supercapacitor is assembled, and a diagram of itsstructure is shown in Fig. 8a. Fig. 8b shows a series of CVcurves of NS@NF-20//rGO asymmetric supercapacitorunder various voltage windows from 0–1.0 V to 0–1.8 Vat 5 mV s−1. The stable window voltage can reach 1.6 Vfor the NS@NF-20//rGO asymmetric supercapacitor.When window voltage increases to 1.8 V, the polarizationof NS@NF-20 electrode leads to the instability for thedevice [45]. Therefore, a potential of 0–1.6 V is selected asthe window voltage to investigate further electrochemicalperformances of the NS@NF-20//rGO device. Fig. 8cdisplays the CV curves of NS@NF-20//rGO asymmetricsupercapacitor at sweep rates from 10 to 100 mV s−1

within 0–1.6 V. The NS@NF-20//rGO asymmetric su-percapacitor exhibits a pair of non-rectangular peaks inCV curves, which indicates a characteristic of pseudoca-pacitance originating from the reversible Faradic reac-tions. Up to 100 mV s−1, CV curves of the NS@NF-20//rGO asymmetric supercapacitor still retain markedly re-dox Faradic reaction peak because the large surface areaof the electrode provides the pathway for ion and electrontransport.

Fig. 8d displays the GCD curves of the NS@NF-20//rGO asymmetric supercapacitor at 2, 4, 8, 12 and20 A g−1. From these GCD curves, high coulombic effi-ciency of NS@NF-20//rGO asymmetric supercapacitor isobserved, which indicates the good electrochemical per-formance. Fig. 8e displays the specific capacitances ofNS@NF-20//rGO as a function of the current densities.The specific capacitance of NS@NF-20//rGO asymmetricsupercapacitor reaches 115.6 F g−1 at 2 A g−1. With up to20 A g−1, it still retains 71.2 F g−1, suggesting the good ratecapability for NS@NF-20//rGO asymmetric super-capacitor. The inset in Fig. 8e shows that two devices inseries light up the red light-emitting diode (LED) for fiveminutes, suggesting the potential application of thisNS@NF-20//rGO device.

The energy and power densities are closely related tothe practical application for energy conversion and sto-rage devices. The energy and power densities of NS@NF-



20//rGO asymmetric supercapacitor are evaluated basedon the GCD data using the Equations (2) and (3). Fig. 8fshows Ragone plot of NS@NF-20//rGO asymmetricsupercapacitor, which exhibits its calculated energy andpower densities based on GCD data. The NS@NF-20//rGO asymmetric supercapacitor shows the excellentenergy density (41.2 W h kg−1 at 1.6 kW kg−1). When itspower density is up to 16 kW kg−1, it still maintains25.3 W h kg−1. The high energy and power densities areattributed to the good rate properties of NS@NF-20 and

rGO electrode materials. This NS@NF-20//rGO asym-metric supercapacitor delivers superior energy densitycompared with the transition metal sulfide electrodematerials in the previous reports, such as the nano-tri-angular Ni3S2@CoS//AC (28.4 W h kg

−1 at 0.134 kW kg−1)[61], CoNi2S4 NSs//AC (33.9 W h kg

−1 at 0.409 kW kg−1)[62], the clustered network-like Ni3S2-Co9S8//AC(17 W h kg−1 at 1.400 kW kg−1) [64], the cliff-like NiO/Ni3S2//AC (43.99 W h kg

−1 at 0.230 kW kg−1) [65], theporous Ni3S2//AC (41.8 W h kg

−1 at 0.155 kW kg−1) [66],

Figure 8 Electrochemical test of the NS@NF-20//rGO asymmetric supercapacitor. (a) Schematic illustration of the assembled NS@NF-20//rGOasymmetric supercapacitor. (b) CV curves at different voltages windows. (c) CV curves at various sweep rates. (d) GCD curves at different currentdensities of 2–20 A g−1. (e) The specific capacitances at different current densities of 2–20 A g−1. The inset shows a digital photograph of two NS@NF-20//rGO asymmetric supercapacitors in series lighting up a red light-emitting diode (LED). (f) Ragone plots of the NS@NF-20//rGO asymmetricsupercapacitor.



Ni3S2/MWCNT-NC//AC (19.8 W h kg−1 at 0.798 kW kg−1)

[67] and Ni3S2/CNFs//CNFs (25.8 W h kg−1 at 0.425

kW kg−1) [68] (the details are shown in Table S4). ThisNS@NF-20 with outstanding electrical properties will be agood potential material for the asymmetry super-capacitors.

CONCLUSIONSNi3S2 NS arrays on Ni foam have been devised and fab-ricated using one-step electrodeposition strategy. TheseNi3S2 NS arrays are interconnected and deposited on thecurrent collector of Ni foam. This cross-linked NS arraystructure is highly beneficial to the transport of ions andelectrons between electrode and electrolyte during thecharge/discharge process. These Ni3S2 NS arrays as anovel electrode material possess excellent electrochemicalperformances for supercapacitors. The Ni3S2 NS arrayselectrode (NS@NF-20) shows 773.6 F g−1 at 1 A g−1 andan 84.3% rate capability from 1 to 10 A g−1, and achieves acycling retention of 81.7% over 5,000 cycles. The as-sembled NS@NF-20//rGO asymmetric supercapacitordevice can reach a 0−1.6 V stable voltage window. ThisNS@NF-20//rGO device also exhibits a maximum energydensity of 41.2 W h kg−1 as well as a maximum powerdensity of 16 kW kg−1. These good performances endowthe Ni3S2 NS arrays electrode material with a morepractical and potential prospects for the asymmetricsupercapacitors. It is suggested that the Ni3S2 NS arrayselectrode material is a new candidate for the commercialapplication of supercapacitors.

Received 1 August 2018; accepted 26 September 2018;published online 30 October 2018

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Acknowledgements The authors acknowledge the financial supportfrom the National Key R&D Program of China (2018YFF0215200), theNatural Science Foundation of Liaoning Province (201602104), theSupport Program for Innovative Talents in Liaoning University(LR2017061), the Basic Research Project of Liaoning Province(LF2017007), and the Scientific Public Welfare Research Foundation ofLiaoning Province (20170054).

Author contributions Xu J conceived the idea of this study andrevised the paper. Sun Y performed the synthesis of the electrode andprepared the manuscript. Liu X revised the paper and coordinated thiswork. The paper was discussed through contributions of all authors. Allauthors have given approval to the final version of the paper.

Conflict of interest The authors declare no conflict of interest.

Supplementary information Supporting data are available in theonline version of the paper.

Jiasheng Xu is currently an associate professor at the College of Chemistry and Chemical Engineering, Bohai University.He got his PhD degree from Dalian University of Technology in 2009. He worked as a postdoctor in Jilin University from2010 to 2012, and worked as a research professor in the University of Ulsan from 2012 to 2013. He got JSPS PostdoctoralFellowship for Research in the University of Tokyo from 2013 to 2015. His current interest is on photocatalysis, lithiumion batteries and supercapacitors.

Yudong Sun received his bachelor degree from Shenyang University of Technology in 2016. He is currently a graduatestudent at the College of Chemistry and Chemical Engineering, Bohai University. His current research focuses ontransition metal based materials for electrochemical energy storage application.

一步电沉积法制备Ni3S2纳米片阵列作为高性能非对称超级电容器的研究许家胜1*, 孙誉东1, 鲁明俊1, 王琳1, 张杰1, 刘晓旸2*

摘要本文采用一步电沉积法制备了Ni3S2纳米片阵列超级电容器电极. Ni3S2纳米片彼此互连能够为电子传导提供快速通道, 有利于电子与离子传输, 提供了丰富的赝电容反应位点. 采用不同电沉积次数探究了不同负载量的Ni3S2对其电化学性能的影响. 性能最好的Ni3S2电极在1 A g−1下展示出773.6 F g−1的单位比电容, 在10 A g−1时具有84.3%的优异倍率性能. 组装的非对称超级电容器(Ni3S2//rGO)表现出优良的使用性能. 这些结果表明了所制备的Ni3S2超级电容器电极材料具有广阔的应用前景. 电沉积法控制Ni3S2负载量的策略能够为电极材料制备提供一种新思路.



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One-step electrodeposition fabrication of Ni3S2 nanosheet arrays on Ni foam as an advanced electrode for asymmetric supercapacitors INTRODUCTION EXPERIMENTAL SECTION Reagents Fabrication of Ni 3S2 electrodesMaterials characterization Electrochemical measurements

RESULTS AND DISCUSSION CONCLUSIONS

One-step electrodeposition fabrication of Ni nanosheet ... · supercapacitor using the Ni 3 S 2...

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